Weak black liquor produced in a wood pulping process typically has a solids content of approximately 15% by weight, which is too low for combustion. To raise the solids content of weak black liquor, the weak black liquor is typically concentrated in multi-effect evaporators until its solids content is approximately 70-90%. Thereafter, the concentrated weak black liquor is referred to as black liquor. Concentrated black liquor is then directed to a recovery boiler where the black liquor is burned and in the process produces ash.
In the design of newer pulp mills, there is a desire to direct the weak black liquor from the digester to the evaporation system without cooling below its atmospheric flash point. This has the advantage of lowering the steam usage in the evaporation system. Current technology uses flash tanks or liquid-to-liquid heat exchangers to cool the weak black liquor prior to reaching the evaporation system. There are drawbacks and disadvantages to both approaches. First, the flash tanks are prone to foaming. The liquid-to-liquid heat exchangers, on the other hand, are prone to plugging with fiber. In addition, liquid-to-liquid heat exchangers also require a large volume of condensate or cooling water to cool the weak black liquor.